The number of workers in part-time jobs jumped by almost 100,000 to nearly nine million in the year to last September, new figures have shown.
Full-time employees rose by 30,000 to 18.4 million, said the Office for National Statistics.
The South West has the highest number of part-time workers, at 37% of the total
London has the highest number of workers of any region of the UK, at 4.4 million, while Northern Ireland has the lowest (700,000).
The biggest fall in employment levels between 2011 and 2012 was in Scotland - down by 34,000 to 2.3 million - although Yorkshire and the Humber, East Midlands, the South West and the North East also had fewer people in employment over the 12 months.
London saw an increase of 143,000 employees and there were smaller rises in the South East, West Midlands, East of England, North West and Wales.
The health sector has the biggest number of workers at 3.7 million, while agriculture, forestry and fishing has the smallest at 200,000, the figures showed.
The biggest fall in employment was in public administration - down by 36,000.
TUC general secretary Frances O'Grady said: "While most part-time workers want shorter hours, the drop in skills and status that accompany it are less welcome. However, much of the recent growth in part-time work has been fuelled by economic stagnation and a lack of full-time job opportunities.
"Britain has the second biggest part-time workforce in Europe but it is beset with problems. For many skilled and experienced women, reduced hours at work can spell the end of their career and a massive salary drop. This is hugely frustrating for individuals and a massive waste of talent that our economy can ill afford to lose out on.
"With an average hourly wage of just £8.01 an hour, Britain's part-time workers need to be better paid. Otherwise, our growing part-time workforce will lead to more in-work poverty."